The present invention relates to a printing device for reproducing information comprising a movable image-forming element having a dielectric surface and an image-forming station in which a magnetic roller with a rotatable electrically conductive non-magnetic sleeve is disposed near the surface of the image-forming element. Means are provided to generate an electric field between the image-forming element and the magnetic roller in accordance with an information pattern. An electrically conductive magnetically attractable toner powder is presented into the zone between the magnetic roller and the image-forming element.
It is known to generate a magnetic field in that zone by using a stationary ferromagnetic knife blade disposed inside the sleeve of the magnetic roller and held between like poles of two magnets.
For example, in European Patent Application No. 191 521, a toner brush formed at the knife blade between the magnetic roller and the image-forming element is described which is not of a constant shape, but continuously varies to some extent. The small variations in the brush shape are caused by variations in the toner power forming the brush, e.g., variations in particle size, particle size distribution and magnetic properties of the toner particles, and variations in the density (quantity) of toner powder in the toner brush. The changes of shape of the toner brush result in changes of shape and location of the toner brush boundary line as seen from the side where the image-forming element leaves the toner brush. Consequently, image faults occur during the image-forming process due to the fact that toner particles are not deposited in the correct place on the image-forming element.
Other types of magnetic developer brushes have been proposed, see for example, U.S. Pat. No. 4,354,454 wherein a brush having a plurality of magnetic portions is described. In Japanese Application No. 59-224369 a device for developing electric charges on stripe electrodes of an electrode drum is shown. This device has the same type of problem associated with device described in European Application No. 191 521.
Accordingly, it is an objective of the present invention to provide a printing device which overcomes image faults described above.